1. Field of the Invention
The present invention relates to a liquid crystal display panel and a method for producing the same. More particularly, the present invention relates to a method for producing a liquid crystal display panel by using one drop filling technique, and a liquid crystal display panel suitable for such a production method.
2. Description of the Related Art
In recent years, as liquid crystal display panels become larger in size, it is becoming more common to adopt a so-called one drop filling technique as a method for forming a liquid crystal layer between a pair of substrates, instead of the conventionally-used vacuum injection technique.
A process of producing a liquid crystal display panel using a vacuum injection technique comprises the following steps.
(a1) On one of a pair of substrates (which typically are a color filter substrate and a TFT substrate) for composing a liquid crystal display panel, a predetermined pattern of sealant is formed. Thereafter, the pair of substrates are attached together, and the sealant is cured, whereby a liquid crystal cell is obtained. The sealant pattern is formed so as to define a region in which a liquid crystal layer will later be formed (note that a display region is encompassed within this region) and also to define an injection inlet.
(a2) After evacuating the liquid crystal cell to create a vacuum therein, a liquid crystal material is injected while keeping the liquid crystal material in contact with the injection inlet, by utilizing a pressure difference between the interior and exterior of the liquid crystal cell and also utilizing capillary action.
(a3) Thereafter, the injection inlet is sealed with an encapsulant.
On the other hand, a one drop filling technique comprises the following steps.
(b1) On one of a pair of substrates, a sealant pattern is formed so as to surround a region in which a liquid crystal layer is to be formed, and thereafter a liquid crystal material is dripped into the region enclosed by the sealant pattern.
(b2) Then, the substantially is attached to the other substrate, and thereafter the sealant is cured.
In the one drop filling technique, the sealant pattern needs to completely enclose the region in which to form a liquid crystal layer. Therefore, when a seal pattern is drawn by means of a dispenser or the like, at least one junction will always be formed.
With reference to FIG. 21, the characteristics of a seal pattern used in the one drop filling technique will be described.
FIG. 21 illustrates an example where four liquid crystal display panels are to be formed from a mother substrate. A color filter mother substrate 20 includes four color filter substrates. In a display region 24 of each resultant color filter substrate, color filters which are arranged so as to correspond to the pixels and a counter electrode(s) (which are not shown) are provided. Each color filter substrate further includes a light shielding layer (black matrix) 22 surrounding the display region 24, such that the light shielding layer 22 defines an outer periphery of the display region 24. Although FIG. 21 only illustrates discrete TFT substrates 10 corresponding to the respective liquid crystal display panels, it will be appreciated that a mother substrate including four unseparated TFT substrates 10 (in a similar fashion to the color filter mother substrate 20) is to be attached to the mother substrate 20 before cutting. In a display region 14 of each of the four TFT substrates 10, necessary circuit elements, e.g., TFTs, pixel electrodes, gate bus lines, and source bus lines, are formed. Each TFT substrate and each color filter substrate are attached together by means of a sealing portion 32. The sealing portion 32 is formed on the outside of the light shielding layer 22.
The portion of the liquid crystal display panel lying outside of the display region 14 is referred to as a “non-display region” or “frame region”, which is expected to be as narrow as possible. The light shielding layer 22 and the sealing portion 32 are provided in the non-display region.
On the other hand, the light shielding layer 22 needs to have a certain thickness in order to prevent unnecessary light from entering into the display region 14. If light shielding is insufficient, black displaying quality is degraded, thus substantially affecting the image quality. In order to satisfy both of these requirements, it is necessary to accurately form the sealing portion 22 at the close vicinity of the outer periphery of the light shielding layer 22.
However, when the sealing portion 32 is formed by a drawing technique, at least one junction 32b will inevitably be formed. The sealing junction 32b tends to become thicker than a main stretch 32a of the sealing. As used herein, the sealing main stretch 32a refers to a portion of the sealing portion 32, excluding the junction 32b, that has a substantially constant width. The main stretch 32a is a portion of a seal pattern that is created with a sealant which is discharged while a nozzle of a dispenser or the like undergoes a relative movement within the plane of the substrate, and therefore depends on the discharged amount of sealant and the moving speed of the nozzle. Thus, the main stretch 32a has a fairly stable width. On the other hand, the junction 32b includes a portion at which the sealant is first applied (i.e., a start point of the seal drawing). The amount of sealant which is initially added depends on the amount of sealant which resides at the tip of the nozzle. The amount of sealant residing at the tip of the nozzle fluctuates due to fluctuations in the length of time spent for positioning the nozzle (including positioning along the height direction), and fluctuations in the amount of sealant which remains at the tip of the nozzle when the nozzle is lifted off the substrate at the end point of seal drawing. Thus, due to the inconstancy in the amount of sealant applied at the start point and end point of seal drawing, and the need to form a junction, the width of the junction 32b tends to become thicker than that of the main stretch 32a. 
FIGS. 22A and 22B are enlarged views showing the neighborhood of a sealing junction. FIG. 22A is a plan view, and FIG. 22B is a cross-sectional view.
As described above, if the seal pattern has a broader width at the junction 32b, a portion thereof may overlap the light shielding layer 22. Since photocurable resins (including those types of photocurable resin which also allow auxiliary use of thermosetting) are widely used as the sealant, if light irradiation (typically ultraviolet (UV) irradiation) is performed from the side of the color filter substrate 20, the portion 32′ of the sealant which overlaps with the light shielding layer 22 cannot be sufficiently cured. As a result, the uncured component of the photocurable resin may elute into the liquid crystal material, thus causing deterioration in reliability, e.g., lowering of the voltage retention rate of the liquid crystal display panel mainly due to ionic components, and occurrence of orientation defects.
It might be possible to perform light irradiation from the side of the TFT substrate 10. However, as will be described later, if UV irradiation is performed from the side of the TFT substrate 10, it becomes necessary to employ some means for preventing the TFTs from being irradiated with UV (e.g., a mask for shielding the TFTs from light must be used). On the other hand, if UV irradiation is performed from the side of the color filter substrate 20, there is an advantage in that such means can be omitted in the case where the color filter sufficiently absorbs UV. At least the amount of damage to the TFTs can be reduced.
In order to prevent partial overlapping between the junction 32b and the light shielding layer 22, Japanese Laid-Open Patent Publication No. 2002-122870 discloses a method which involves drawing a seal pattern that extends to the outside of the liquid crystal display panel, and forming a junction 32d outside of the liquid crystal display panel, as shown in FIG. 23. Moreover, as shown in FIG. 24, Japanese Laid-Open Patent Publication No. 2002-122870 discloses a method which involves forming a start or end point 32e of seal drawing outside of the liquid crystal display panel.
Japanese Laid-Open Patent Publication No. 8-240807 discloses a method which involves forming a sealing junction at a corner portion, taking advantage of the fact that the interspace between a sealing portion and a light shielding portion will be about 1.4 times greater at a corner portion of the liquid crystal display panel than along any side thereof. However, this method cannot cope with the above problem once the width of the sealing junction exceeds about 1.4 times the aforementioned value, as is also pointed out in Japanese Laid-Open Patent Publication No. 2002-122870.
If the method described in Japanese Laid-Open Patent Publication No. 2002-122870 is used, it is no longer necessary to form a junction 32b within the liquid crystal display panel. However, as shown in FIG. 25A, this method is only applicable to the case where, after a TFT substrate (TFT mother substrate) 10 is attached to a color filter substrate (CF mother substrate) 20, both mother substrates 10 and 20 are cut along the same line CL. There exist other cases where, as shown in FIG. 25B, the TFT mother substrate 10 is cut along a cut line CL1 and the CF mother substrate 20 is cut along a cut line CL2 different from the cut, line CL1, this being in order to provide a signal line terminal section (driver mounting portion) on the TFT substrate 10. In such cases, there is a problem in that a sealing portion 32t will remain on the TFT mother substrate 10, which will stick to a fragment of the CF mother substrate 20 to be removed, thus making it impossible to remove the fragment. Note that in a structure where signal line terminal sections are provided along three or four sides of the liquid crystal display panel in order to suppress signal delays and the like associated with an increased size of the display panel, it is impossible to adopt the seal pattern as shown in FIG. 25A. Although the pattern shown in FIG. 25A can be adopted in the case where signal line terminal sections are provided along two sides of the liquid crystal display panel, the need to cut the sealing portion 32 concurrently with the mother substrates 10 and 20 may invite cutting failures.
Furthermore, the method described in Japanese Laid-Open Patent Publication No. 2002-122870 will require an apparatus which is able to draw a seal pattern to the outside of a liquid crystal display panel without allowing a junction to be formed before going out of the display panel. Such a seal pattern drawing apparatus will inevitably be large in size, and hence increase the production cost of the display panel.
According to a study conducted by the inventors of the present invention, the method described in Japanese Laid-Open Patent Publication No. 8-240807 will have not only the problem mentioned in Japanese Laid-Open Patent Publication No. 2002-122870, but also another problem in that, as schematically shown in FIGS. 26A and 26B, the width of the sealing junction may occasionally exceed the 1.4 times value at a corner portion, even without forming a junction. In such cases, the sealant 320 present in the overlapping portion with the light shielding layer 22 cannot be sufficiently cured.
In addition to sealing junctions and corner portions, similar methods may also occur in any transfer section for establishing electrical connection between the upper and lower substrates (e.g., a common transfer section).